The Baofeng series of radios – our recommended best choice for most prepping purposes.

The General Mobile Radio Service – or GMRS for short – has evolved since its original establishment in quite different form in the 1960s.  In 1987 it evolved to essentially its present form, with business use specifically excluded, with a set of eight channel pairs for repeater operation, and seven single channels for non-repeater ‘simplex’ operation.

In 1996 the FCC created a new product – the Family Radio Service or FRS for short.  This was sort of ‘GMRS-lite’ and almost literally was squeezed into the ‘gaps’ between GMRS channels.  It was designed as a low power service for handheld walkie-talkie type radios only, and unlike GMRS there was no licensing requirement.  Anyone could buy FRS radios and ‘play’ with them more or less as they wished.

Because the FRS and GMRS frequencies were sometimes the same and sometimes only very slightly different, radio manufacturers started selling dual-purpose radios that could work either as FRS or GMRS radios.  In theory, this was illegal, but in practice, it was so widespread that the FCC chose to do nothing.  This created a confusing mix of frequencies and channel numbers, and we previously explained the implications of this in our earlier article explaining the confusion of frequencies between FRS and GMRS.

Finally – 21 years after this all started to become a mess, the FCC has acted to try and clear things up, publishing new rules and frequencies in September 2017, taking effect from late October 2017.

Unfortunately, things are still far from clear, but they are perhaps slightly less messy than before.  In particular, there will soon no longer be any more dual purpose radios – something we discuss in our article explaining the changes in FCC regulations about FRS and GMRS radios.

Here now is the new improved official list of channels and an explanation of what each channel can be used for.

Let’s first look at the most relevant list of channels for many of us – the FRS channels.

Table 1 :  FRS Channels and Power

Note that all FRS channels require a radio with NFM – ie, 12.5 kHz bandwidth.  And, as you can see, the numbering of the channels seems somewhat illogical and out of sequence with the order of the frequencies, as does the different power requirement for the 467 MHz frequencies.

Note also that the FCC assigns official channel numbers to these channels.  Whether new radios will follow the official channel numbering or not is anyone’s guess, but the list, as above, is the ‘gold standard’ FRS frequency/channel list taken directly from the FCC’s own website.

The FCC believes it all makes sense, and helps to fit the FRS frequencies more consistently into the GMRS frequencies too.  So let’s now look at the GMRS frequencies.

In the case of GMRS, not only do we show the frequencies and power, we also show what types of GMRS radio can use each frequency, and explain the frequency pairs for repeater use.  On the other hand, there are no official channel numbers.  Yes, it is quite a lot messier than the simple table above.

Table 2 :  GMRS Frequencies and Conditions

As you can sort of see from this table there are several differences between the GMRS and FRS services.  Although the FRS channels 8 – 14 are essentially identical with both services, the FRS channels 1 – 7 are restricted to 2 watts whereas the same GMRS channels allow 5 watts, and the channels 15 – 22 can go up to 50 watts with GMRS service but only 2 watts with FRS.

Plus FRS radios are limited to small handheld radios with fixed antennas, whereas the GMRS radios can have external antennas and be ‘base’ stations with greater power and/or repeaters.

In the power column, we show power of 50/15 for some frequencies.  In these cases 50W can be used for mobile, base and repeater radios, but only 15 watts for fixed stations.

So what are these different types of radios?  In the usage column we list them as :

B :  A Base radio station – a station located at a fixed location that communicates with other base and mobile stations

C :  A Control radio station – a base station that communicates through repeaters and can control the repeater

C* :  A Control radio station transmitting through a repeater

F :  A Fixed radio station – a base station that only communicates with other fixed stations

H :  A Handheld portable radio

M :  A Mobile radio station (ie in a vehicle)

M* :  A Mobile radio station transmitting through a repeater

R :  A Repeater radio station – ie one that receives (in this case) on one of the 467 MHz 50 watt channels and automatically/simultaneously retransmits it on the 50W channel that is 5 MHz lower (ie the matching 462 MHz channel)

Note – these definitions are per the FCC Part 95 regulations and seem a bit strange.  On the face of it, it seems that handheld units can’t communicate via repeaters, but we suspect this is not actually the case.

Simplifying the GMRS Frequencies

The key difference between the GMRS and FRS frequencies is there are an added 8 GMRS frequencies that are intended to be used only for repeater inputs (in the 467 MHz range).

Otherwise, the frequencies are the same, but power levels are different, and also in the high power channels, GMRS can use wideband FM as well as narrowband FM.

Most of these differences are automatically taken care of in the radio sets themselves.

The strange mix of frequencies sort of makes sense with this graphic illustration published by the FCC.

The FCC’s pictorial explanation of the new FRS and GMRS frequency allocations, which still quite literally leaves gaps or holes in their frequency allocations and reasoning

Summary

The new FRS frequencies and power allocations are much better than the original 14 frequencies and 0.5W power limit.  This is a good reason to get new FRS radios, and/or to reprogram your Baofeng or other radios to conform to the new frequencies and power limits.

It is possible the FCC may now start to lightly enforce some of its rules for the FRS and GMRS bands, so it would be wise to generally comply with the new requirements, now they are in effect.

Lastly, and as mentioned in the other article about the 2017 FCC changes to FRS and GMRS, you might prudently choose to get some more Baofeng or similar multi-purpose radios prior to their probable banning, early in 2019.

An example of a signal strength reading from a $25 SDR receiver and using free software on your PC. Essential for testing your Faraday cage.

So you’ve built your Faraday cage, and are ready to fill it with the precious electronic spares you want (need!) to protect against an EMP.  Well done.

But there’s one thing you really should do before closing it up and moving on to your next project.  Even if you’ve built a simple small and apparently 100% compliant cage, don’t you think you should check and test its effectiveness?  You’re placing a huge amount of reliance on this cage and its ability to protect its contents from an EMP, so isn’t it worth spending a bit of time and maybe a few dollars to check that it works as expected.

The interesting thing is that a Faraday cage is not an ‘all or nothing’ thing.  It isn’t like putting heavy blackout curtains across your windows and blocking out all the light from outside.  It is more like pulling a medium weight curtain across, and then having a searchlight shine on your window from outside.  Some light will get through, the key issue is how much.

The same with a Faraday cage.  It will reduce the strength of the EMP (the technical term is ‘attenuate’) but it won’t block it out entirely.  So how much attenuation will it provide, how much do we need, and how do we measure it?

How Much Attenuation is Desirable

First, how much attenuation do you need?  More is always better than less, but after a point, you reach a realm of vanishing returns and unnecessary extra protection.  We suggest you should look for 40 dB (dB = decibel) of attenuation, and ideally over 50 dB.

What do these numbers mean?  If you reduce something by 30 dB, you have reduced its power 1,000 times.  If you reduce it by 40 dB, you’ve reduced its power 10,000 times.  If you think to yourself ‘every ten dB adds another zero to the number of times the signal is reduced’ then you’d be correct.

So, after the lesson, the test.  How many times would the strength of an EMP be reduced if it had a 50 dB attenuation?  Please tell me that you answered 100,000 times.  Clearly that’s a lot, isn’t it, and probably it is enough.  Let’s see why we say that.

A strong EMP effect will induce voltages in the order of 50,000 volts/meter (it is difficult for EMPs to exceed this – anything higher than that and the sky sort of ‘short circuits itself’, although there are rumors of some ‘super EMP’ weapons that have found a way to create higher voltages).  If we reduce 50,000 by 40 dB, it is down to a trivial 5 volts/meter, and if we take it down by 50 dB, it is down to a hard to measure 0.5 V/m (ie 500 mV/m).

To put this into context, a strong radio signal ranges between 1 – 100 mV/m.  So after about 56 dB of attenuation, the strongest probable EMP is no more harmful than a strong radio signal – ie, totally utterly harmless.  And we’ve at least 10 dB of overload above that and probably 20 dB before there needs to be significant concern about EMP damage to electronic circuits that are switched off.

Measuring Attenuation – Decibels and Frequencies

There is an interesting complicating factor.  An EMP has a mix of different radio frequencies, and Faraday cages block different frequencies by varying degrees.  In general terms, the higher the EMP’s frequency, the less the attenuation.

So that begs the question – exactly what frequencies are likely to be found in an EMP?  The short answer is ‘all of them’, which isn’t very helpful, is it!  A better answer is ‘most EMPs are expected to concentrate most of their energy in frequencies below 100 MHz’.  To put 100 MHz into a meaningful context, it is right in the middle of the FM dial.

That is actually a bit of welcome good news.  The lower the frequency, the better most Faraday cages work at blocking it.  So if we test the cage at 100 MHz, we know that its blocking will be better at lower frequencies and that the 100 MHz result is getting close to a ‘worst case’ scenario.

There are a number of ways you can do this test.

Test 1 – The Phone Test

This is the easiest of all the tests, and is also the least valuable, but as a quick rule of thumb test, it can tell you if your Faraday cage might fail, although it probably can not tell you if the cage for sure will succeed (testing for failure and testing for success are surprisingly different things).

Take your cage and your cell phone to somewhere with good cell phone coverage.  Ideally, go somewhere where the cell phone is receiving on the approx 850-900 MHz range of frequencies, rather than on the 1900 MHz set of frequencies.  You can do this by using the excellent antennasearch.com website – put in your address, and then download all the different transmitters it finds close to you (chances are you’ll be surprised at how many there are!).  Use the tower map to find a tower belonging to the wireless service you use and which has only 900 MHz not 1900 MHz transmitters on it, then go to it to test.

This is more difficult than it sounds, and probably you’ll compromise by simply testing at home.  That is okay, but to get a helpful result, make sure that your cell phone has four or five bars of signal strength before putting it in the cage.

Close up your cage, and call your phone.  Maybe you’ll be able to hear the phone ringing inside.  If the phone rings, you know your cage is probably not working well.

If the cage material insulates the sound, simply check to see if the phone reports a missed call when you open it up again.  If it shows a missed call, that suggests your cage isn’t fully optimized.

Your cell phone can probably receive signals down to a signal strength of about -115 dBm.  You can probably get your phone to tell you the strength of the signal it is receiving if it is an Android or iPhone – here’s a good article on how to do so, and if you’re interested, this article tells you more.

So if your phone was showing a signal strength of -85 dBm before it went into the cage, and it didn’t ring, you know that the cage is probably providing at least 30 dB or more of attenuation (ie dropping it from -85 to -115 or more) but you don’t really know how much more than the 30 dB it is giving you.  And with only 30 dB of attenuation, that is close to a ‘failure’ so not hearing the phone ring, in this case, isn’t as positive as you’d think.

And if your phone is showing -55 dBm before going into the cage and still rings, you know the cage is providing less than about 60 dB of attenuation, but you don’t know how much less.  At 60 dB of attenuation, having the phone ring can still be considered a ‘success’, although you mightn’t think so by hearing the phone ring

So you need to interpret your phone results with care and caution.  You may be getting either false positive or false negative results; it helps if you know the signal strength the phone was receiving before going into the cage, and what the minimum signal strength is your phone will work on.

Plus, you are ‘torture testing’ your cage by checking its effectiveness at 900 MHz or 1900 MHz.  There is unlikely to be significant energy in an EMP at those sorts of frequencies, and the cage will probably offer better protection at lower frequencies.

So let’s now look at a slightly more helpful test.

Test 2 – Two Walkie Talkies

If you can walk inside your cage, go in with a walkie-talkie and have someone outside with a second one.  Close the cage up and see if you can communicate between the two of you.  If you can, that is suggesting a ‘fail’; if you can’t, that is suggesting a ‘pass’ for your cage.

The walkie talkies ideally should be on the MURS frequencies (about 150 MHz).  If you don’t have MURS capable radios, then FRS or GMRS (about 450 MHz) will work in a pinch, as well.  CB radios would not give as meaningful a result.

If you can’t walk inside your cage, that’s okay too.  In slightly simplistic terms, a cage not only blocks external signals from traveling into the cage, but also blocks internal signals from traveling out.  So all you need to do is use a rubber band to hold down the transmit key on a walkie-talkie, put it into your cage and close it up, and see if you are receiving any signal on the other walkie-talkie outside.

Note that walkie talkies sometimes have a ‘timer’ that stops their transmitting after a period of time.  Check that, when you reopen the cage, the walkie-talkie inside is still transmitting.

This is a good test with fewer ambiguities than the cell phone test.  You can be reasonably certain that if you can’t get a walkie-talkie signal through your cage, then it will block an EMP too.

But if the radios can still communicate with each other, is that necessarily a fail?  It depends on how much the signal strength has dropped.  If one of the radios has an accurate S-meter on it, this is helpful.  In theory, each ‘S unit’ on an S-meter represents a signal strength change of 6 dB, so you would want to see a 7 S unit movement or more between the two signals.  However, particularly, with inexpensive walkie talkies, they either have no S meter at all, or the S meter they do have is woefully inaccurate.

It is not prudent to rely on S meter values, even on quite expensive receivers.

Test 3 – Walkie Talkie and SDR

Good news.  You can actually do a very accurate test of the efficiency of your cage with very inexpensive items.  You want to have a walkie-talkie to place inside your cage, and use a SDR to measure the signal strength, accurately, on the outside.

For the walkie-talkie; if you don’t have a bunch of them already, get a Baofeng UV-5R.  While it is some years since we reviewed these lovely units, and while there have been later models released, the standard UV-5R, at a cost of around $30 each on Amazon, still remains an unbeatable value and excellent performer.  Tune it to a frequency around 145 – 155 MHz, it doesn’t really matter what.  You could tune it down to a lower frequency (they work down to 136 MHz) but best to keep it around the radio’s ‘sweet spot’.

But what is an SDR?  The acronym stands for ‘Software Defined Radio’.  Instead of a traditional radio with knobs and dials and everything, a SDR is a computer device – often in the form of a USB stick that connects into your computer, and which is then controlled by a computer program rather than by old-fashioned ‘analog’ controls.

These are amazing gadgets that can be had for as little as $25 (actually, you can get ones for even less than $25, but this specific $25 SDR is the best performing/best value unit until you start going way over $100).  Download the free SDR+ (pronounced ‘SDR Sharp’) software from here, plug the unit into your computer, and run the SDR+ software.  (If you have a Mac, you could try this free software.)

SDR radios have many uses for preppers, particularly as broad-band multi-mode scanners.  We’ll write more about them in future articles.

Tune to the same frequency as your HT (walkie talkie), make sure that you’ve zeroed out the RF gain and turned off the AGC, and you can get information on exactly the dBm signal strength that is being received by putting the cursor on the top frequency display.  If the signal is too strong – ie, you don’t get a nice single peak on the SDR’s display, but a whole series of peaks and a general lifting of all the frequencies being monitored, take the antenna off the SDR and maybe replace it with just a very short piece of wire.

Then stick the HT into the cage, close it up, and you’ll see exactly the drop in signal strength.  Maybe you go from +5 dBm to -40 dBm, or something else.  In such a case, clearly the cage has provided you with 45 dB of attenuation.  How easy (and accurate) is that!

In this screen shot (a larger view of the image at the top of the article), you can see that the display is monitoring a signal at 145.330 MHz, with a signal strength of +2.5 dBFS (forget about the FS, it means ‘relative to full scale’ – a concept which is not relevant to our needs).  That is a very powerful signal, and as you can see, it ‘stands up’ a long way above the random background ‘noise’ (the other data tells us the background noise is 56.2 dB below full-scale, and so in total, there is a separation of 58.7 dB between the background noise and the measured signal) and some other weak signals that you see small peaks for.

After getting your ‘out of cage’ reading, which this is, you then simply put the radio in the cage and get a second ‘in the cage’ reading.  The difference between the two dBFS readings is the amount of attenuation, and hopefully it is reporting more than 40 dB.

Summary

You shouldn’t assume your Faraday cage is giving you the protection you need and are relying on.  Fortunately, there are some relatively easy ways to test its functionality.

Ideally, you should be getting more than 40dB of attenuation from your cage.

The TYT TH-9800 sets a new high-water mark for excellent value feature-filled two-way radios.

We’ve written before about how wonderful radio repeaters are for extending the range you can communicate with your two-way radios.  No matter what you do to your portable and in-vehicle radio transmitters and receivers, sooner or later you will probably have to consider using a repeater to extend your communications to reliably cover the region you need to be able to keep in contact with.

But maybe you are very fortunate, and your terrain/topography is such that you have reliable radio comms all over your retreat and adjacent areas.  Does that mean you don’t need to think about repeaters any further?

Maybe.  But also, maybe not.  There are two other applications for repeaters that you might want to consider.  Please read on and see if either (or both) might apply to you.

Vehicle Range Extenders

This is something that could be of value to many preppers.  Maybe you have a heavy-duty high-powered radio in your vehicle, and a fully optimized antenna mounted on it as well.  Maybe that gives you the range you need to be able to communicate reliably everywhere you are likely to be, and with everyone else who you might need to communicate with.  But what happens when you step out of your car and switch from the 50 watt (maybe more) unit in your car to a 5 watt (probably less) unit in your hand, from a full size antenna on top of the vehicle roof to a short stubby little thing in your hand?

Quite likely you will find yourself unable to punch out or pull in the signals you need, and you’re now in a position of dual vulnerability.  Out of your car, you’re less protected, and you’re also out of contact with the rest of your community.

You’re not the first person who has struggled with that problem.  It used to be an issue for police officers, the world over.  Now, not so much, because there is a solution that they’ve widely deployed, and which you might wish to consider, too.

This is an in-vehicle repeater.  It uses the high power, high quality circuitry, and optimized antenna on your vehicle not only to send/receive messages to far-away contacts, but it also retransmits the signals it receives, on a different frequency, to allow you to pick up the repeated transmissions more clearly from a hand-held transceiver.  Yes, it is also simultaneously ‘listening’ for the signals you send it from your handheld radio, too.  For you, out of the vehicle, you no longer need the ability to receive signals from people far away, and to be able to transmit back to them.  You only need to be able to receive transmissions from your vehicle, and send back to it.

These days, when you see a police officer talking into his chest microphone, that radio is probably not transmitting all the way to wherever the main receiver tower is, but only to his nearby cruiser.  He is now safely in contact, as long as he is in range of his vehicle.

If you seek a similar capability, and have the appropriate FCC ham or commercial license to operate the radios and repeater, you can do the same.  Many mobile (ie in-vehicle) type two-way radios have what is termed a ‘cross-band repeater’ capability – it is generally easier and less expensive to have the signal that is being repeated to be retransmitted on a very different frequency to that it was received on, which is what ‘cross-band’ means.

These amazing units can be very expensive, but do not need to be, and don’t really cost much more than a normal dual band transceiver.  For some time we have liked the Anytone AT-5888UV dual band mobile radio , currently showing at $313 on Amazon (update, June 2016 – now down to about $250), but we’ve now got a new favorite, the TYT TH-9800 .

This latter radio is not only slightly less expensive, (currently showing at $278 on Amazon – update :  now about $210 in June 2016) but also has four bands in it rather than two.  It has the same 2M and 70cm bands that the AT-5888UV offers, plus also the 6M and 10M bands too.  The 6 meter band is an interesting addition, and – with the right antenna connected – can give considerable extra range compared to the short-range on 2M and 70cm.

Currently the AT-5888UV does have one advantage over the TH-9800.  It is compatible with the CHIRP programming software, and from our perspective, if you can simplify the management of your radios by using the same software interface for multiple radio types, that is an enormous plus.  We hope the TH-9800 will be added to the list of radios CHIRP supports, and because the TH-9800 is relatively very new, it will probably take a while for this to happen.  Update – yes, all good things come to those who wait.  The TH 9800 is also now supported by CHIRP.

Note that you need a radio that has the capability to transmit and receive ‘in both directions’.  If it could only repeat in one direction, you would be able to listen but not transmit (or vice versa) – you need the radio to support the repeating both ways.  The two radios mentioned above would be good choices and have this capability.

Comsec

This is an interesting consideration.  As you know, it is very easy for anyone with a scanner to monitor your radio transmissions, but if you are transmitting on one frequency and listening on another, maybe it becomes harder for a casual person listening on a scanner to hear your complete conversation.

That’s a definite ‘maybe’ because if they pick up the repeater signal, they’ll get both sides of it.  But depending on their location and settings, maybe they’ll only get one side or the other of the conversation.

There’s another side to this coin, however.  If you adopt this approach, you are now doubling the frequencies that are carrying your signals – and the repeater is probably sending them over a wider area.  So is it a good idea or bad idea?  Frankly, we’re unconvinced of its good sense, and mention it here more for the purpose of rebuttal than recommendation.

This concept can be made more ‘semi-secure’ (if there is such a thing) by splitting your transmit and receive frequencies more broadly, perhaps having one in one frequency band and the other in another.  But whatever you do, the repeater signal will be carrying both sides of the conversation – find that, and anyone monitoring can hear the entire conversation.

If you are going to do this type of split transmitting/receiving, you of course need to use a repeater.